A radio communications system includes an uplink (UL) and a downlink (DL). A base station (BS) may send a signal to user equipment (UE) through the downlink, and the user equipment may send a signal to the base station through the uplink. When duplex communication is supported, to avoid mutual interference caused by parallel transmission of signals on the uplink and the downlink, it is necessary to separate an uplink signal from a downlink signal.
At present, a duplex mode used in the radio communications system includes frequency division duplex (FDD) and time division duplex (TDD). In the frequency division duplex mode, different carrier frequencies are used in the uplink and the downlink, and the uplink signal is separated from the downlink signal by using a frequency guard interval, which may implement simultaneous and inter-frequency full duplex communication. In the time division duplex mode, different communication time are used in the uplink and the downlink, and a receive signal is separated from a transmit signal by using a time guard interval, which may implement co-frequency and asynchronous half duplex communication. Compared with time sensed by a user, the time guard interval used in the time duplex mode is extremely short, and the time duplex mode is sometimes considered supporting the full duplex communication.
For example, FIG. 1 is a schematic diagram of a scenario of a radio communications system in the prior art. In the downlink, the base station sends a radio signal S1 at a frequency F1 to the user equipment at a time T1; in the uplink, the user equipment sends a radio signal S2 at a frequency F2 to the base station at a time T2. When the frequency division duplex is used, the frequency F1 and the frequency F2 are different, and there is a frequency guard interval between the uplink and the downlink, which may implement the simultaneous and inter-frequency full duplex communication. When the time division duplex is used, the time T1 and the time T2 are different, and there is a time guard interval between the uplink and the downlink, which may implement the co-frequency and asynchronous half duplex communication.
If the frequency F1 and the frequency F2 are the same, and the time T1 and the time T2 are also the same, when receiving the radio signal S2 sent by the user equipment, the base station also receives a self-interference signal S′1 at the same frequency, where the self-interference signal S′1 may be considered a part of the radio signal S1 sent by the base station. Likewise, when receiving the radio signal S1 sent by the base station, the user equipment also receives a self-interference signal S′2 at the same frequency. Due to fast fading of a radio signal during spatial propagation, strength of a local self-interference signal is generally much greater than strength of a radio signal from a remote end, and when sending signals, the base station and the user equipment cannot accurately receive signals at the same time. Therefore, it is generally considered that the radio communications system cannot support co-frequency and simultaneous full duplex communication until a full duplex technology occurs.
Theoretically, in a radio communications system that uses the full duplex technology, a same time and a same frequency are used in the uplink and the downlink, and spectral efficiency may be doubled. However, the full duplex technology is currently at a research and experimental stage, and how to effectively reduce impact of a local self-interference signal on receiving a radio signal from a remote end is still a critical technical problem that needs to be resolved in the full duplex technology. A current research direction mainly includes two types: one is eliminating a local self-interference signal by signal processing in an RF module; the other is optimization in an antenna to reduce strength of a local self-interference signal that enters an RF module.
In the prior art, design of the signal processing method in the RF module to eliminate the local self-interference signal is mainly considered, and design and optimization for the antenna are not common. In a current full duplex experimental communications system, an antenna system usually separates a transmit signal from a receive signal in a manner of physical isolation between antennas. For example, a physical distance between a receive antenna (Rx antenna) and a transmit antenna (Tx antenna) may be increased so as to improve isolation between the receive antenna and the transmit antenna. However, in this case, a size of the antenna system is relatively large, which hinders device miniaturization and deployment of an actual project. Therefore, an optimization design for an antenna that provides an antenna with good transceiving isolation is of great significance to an application of the full duplex technology in a future radio communications system.